Orbital abrading machines are well-known and generally comprise a portable, manually manipulatable housing, a motor supported by the housing and having or being coupled to a drive shaft driven for rotation about a first axis, and an assembly for mounting a pad for abrading a work surface for orbital movement about the first axis. In a random orbital abrading machine, the assembly serves to additionally mount the pad for free rotational movement about a second axis, which is disposed parallel to the first axis.
The assembly typically includes a head portion coupled for driven rotation with the drive shaft about the first axis and defining a mounting recess having an axis arranged coincident with the second axis, a bearing supported within the mounting recess, and means for connecting the pad to the bearing for rotation about the second axis.
An orbital machine having an element, such as pad, driven for movement about an orbital path of travel is by nature unbalanced and tends to produce vibrations, which may be felt by the hands of an operator of the machine. With a view towards maintaining such vibrations at acceptable levels, it has been common practice to employ a counterbalance system of the type described in Chapter 12 Mechanisms and Dynamics of Machinery, Third Edition, by Hamilton H. Mabie and Fred W. Ocvirk, published by John Wiley and Sons, which is incorporated by reference herein. The aforementioned design approach, commonly referred to as “dynamic” balancing, accounts only for the unbalance which is created by the mass centers of the pad and portions of the assembly not disposed concentric to the first axis. Dynamic balancing adds counterweight masses to the housing that are symmetrically positioned with respect to a radial plane of the second axis.
Dynamic balancing can create a machine that is balanced, that is, has acceptably low vibration levels, while the machine is running at free speed in an unloaded condition. However, once the machine is loaded, as a result of placing the pad in abrading engagement with a work surface, additional forces are introduced and the machine becomes unbalanced. This unbalance is detected by the operator in the form of vibration. This vibration is undesirable and in severe cases, may lead to vibration-induced injuries such as carpal tunnel syndrome and white finger.
An improved design approach shown in commonly assigned U.S. Pat. No. 6,206,771 (Lehman), which is incorporated by reference herein, and which is hereinafter referred to as Lehman, employs counterbalancing in such a manner as to minimize vibrations under actual working conditions. However, the counterbalancing disclosed in Lehman is only effective for predetermined operating conditions.
What is needed then is a convenient and cost-effective means of balancing an orbital abrading machine to minimize vibrations associated with a wider variety of abrading operations.